MP 1,1-5,1 Mac Pro 3,1 GPU Upgrade

[TechInit]

Hi all,

I'm a novice when it comes to power supplies, and how much power specific things can output, so I've come here for help. I've decided to get a GTX 780 for my Mac Pro 3,1, because I want something I could use if I ever upgrade, even if it's bottlenecked slightly, however, that's not the focus of this post. I know the Mac Pro 2008 PSU is rated for about 900w. I know that the 6-pin connectors on the Motherboard are rated for 75w each. With this, this totals 150w, which is the amount an 8-pin connector is rated for outputting. Does this mean I could get a 2xMini 6-pin to 8 pin, and have things run fine? Also Is this advisable, or inadvisable? I also want to know how well a 2xMolex to 6-pin would work. I've heard mixed things, with some people saying its fine, and others saying it isn't. I'd just be needing it for roughly 25w, as the PCI-e slot supplies 75w, alongside the 2x75w from the 6-pins.

Also am I doing this right or am I being a dumb dumb?

(also the 780 is rated for a TDP of 250w)

 

[Macschrauber]

The gtx780 has one 8 pin and one 6 pin ?

Otherwise, if one 8 pin, double mini 6 to one 8 is fine.

 

[TechInit]

Its true the GTX 780 has both an 8 pin and a 6 pin, and I'm glad to hear that the 2x6-pin to 8 pin will work fine, I was just also wondering that for the extra 25w that's needed, am I alright to use a 2xMolex to 6-pin, as I have no other method of getting the extra required power?

Thanks for the help!

 

[mikas]

It's not likely that the card draws 75 watts from the PCIe slot, they very rarely do that. You will have to assure there is enough power for the 6-pin connector too in your card, and it could be as much as 75 w - not 25 w. I haven't tried to use those CD/DVD molex connectors for extra power myself. I have used SATA power though, but that is a different specification.

It would be easier to buy a 2x6 pin card, like many of the GTX 680 are. I've got a GTX 680 4GB (Mac flashed) with 2x6pin and just about to install it in 3,1.

 

[KeesMacPro]

A while ago I've been using a GTX 780 in my MP 3,1 until I replaced it by a MP 4,1>5,1.
The GPU was powered by 1 mini6pin> 8 pin and 1 mini6pin> 6 pin cables.
I've never had any issues.


AFAIK the GTX 780 TI consumes more power and I still got one installed in one of my MP4,1>5,1 powered by PIXLAS Mod.

 

[Macschrauber]

Ive measured overclocked 680 with 8 and 6 pin max load with Furmark and it wont got over 90 W on the 8 pin.

IF so a mini 6 to 8 and a mini 6 to 6 is ok. IF...

Better would be load balancing like this

double mini 6 to 8 and

Adapter 8 to 2x 6plus2

So both rails are parallel

 

[mikas]

Nvidia GeForce GTX 780 Review: Titan’s Baby Brother Is Born

At $1,000, GeForce GTX Titan only made sense for folks building small form factor PCs and multi-GPU powerhouses. Now there's another option with every bit of panache, a slightly de-tuned GPU, and a price tag $350 lower: meet Nvidia's GeForce GTX 780.

www.tomshardware.com

In torture test gtx 780 can go up to 253 W peak value.

If the card draws at that peak lets say something like:
33W from slot
70 from 6-pin
150 from 8-pin

total 253 W

With two molex connectors it would leave a stress of 35 watts per connector. They seem to be specced at 50 W. I don't know but 4-pin molex are said to be subject to arcing too.

We don't know how power draw is spread among connectors though, it might look something totally else than my guess above, like:

15W from slot
88 from 6-pin
150 from 8-pin

or

40W from slot
43 from 6-pin
170 from 8-pin

Some of those scenarios would be ok with 2x molex, some of them wouldn't so much. I would not risk it without knowing exactly what is happening. Cards do differ with each other too, by manufacturer, by memory configuration etc.

I am using 2xSATA power for my Red Dragon Vega 56s 6-pin connector. The card is set at low power bios, and I have measured the wattage with software. I have been using the combo from january - february aka over 6 months, also stressing it regularly with twinmotion and sometimes with prorender too. No games though.
Jan 22, 2020
Jan 23, 2020
In my case with my card the wattage per SATA connector is about 16 watts (2xSATA to 6-pin). SATA power is specced at 54 W. There will be spikes though, and they might break something. So you want to have some kind of overhead in there all the time. Mar 14, 2020

There are some power spikes recorded in my mixed measurements Mar 27, 2020 .
It's important to consider those spikes.

Of course it all depends on usage too. One might get away with a lot less power draw too if usage is light. But I wouldn't take that change, but obtain a less power hungry card, if there is no real need for performance to start with.